Generally, technologies are known that remove particulate matter (hereinafter referred to as PM) and nitrogen oxide (hereinafter referred to as NOx) contained in exhaust gas exhausted from an internal combustion engine, such as a diesel engine.
Among the technologies, as a technology to remove NOx, an exhaust gas purification device that uses a selective catalytic reduction (SCR) system is known. The device injects a reducing agent at an upstream side of a NOx catalyst arranged in an exhaust gas passage, and causes the reducing agent to flow into the NOx catalyst together with exhaust gas, thereby effectively decomposing Nox (NO and NO2) in the exhaust gas to nitrogen and the like and discharging them.
As a form of the exhaust gas purification device described above, a muffler has been proposed that functions to purify NOx in exhaust gas, and has a structure in which the flow direction of the exhaust gas is forcibly returned in the muffler. More specifically, a muffler 401 with an exhaust gas purification function is disclosed (refer to Patent Document 1) in which, as shown in FIG. 8, an oxidation catalyst 411 and an injection nozzle 412 that injects and supplies a liquid  reducing agent are arranged in an upstream side exhaust gas passage 409. A NOx reduction catalyst 413 and a reducing agent oxidation catalyst 414 are arranged in a downstream side exhaust gas passage 410. A ring member 415 having holes 415a formed in a periphery thereof is provided in a return portion 408. Thus, the exhaust gas is introduced downstream from the holes 415a, and the exhaust gas flow is generated along a return portion wall surface 408a so as to produce a vortex on a vertical plane of the return portion 408.
Patent Document 1: Japanese Patent Application Publication No. JP-A-2005-273580 (Claims, FIG. 1)